370 VETERINARY PHYSIOLOGY 



determined when a complete detailed account is kept of the 

 intake and output of both energy and material. This can be 

 done by the use of the calorimeter (p. 259), which registers 

 the amount of energy liberated, and also the consumption of 

 oxygen and the output of carbon dioxide in the expired air. 

 The additional information obtained by analysis of the 

 collected urine and faeces give a complete account of the 

 sum of the changes in the food taking place within the body. 

 The energy and material intake in the food can be compared 

 with the energy and material output and the gain or loss 

 determined. The comparatively few complete balance ex- 

 periments which have been done on large animals have 

 afforded valuable information as to food requirements and 

 the productive value of the different feeding stuffs and their 

 constituents. 



Protein Requirements. — Protein beyond the requirements 

 for construction and repair of tissues is catabolised yielding 

 energy, the nitrogen being excreted in the urine. By sub- 

 stituting carbohydrates and fats as a supply of energy the 

 protein intake may be reduced. When the intake is reduced 

 below the level of the protein requirement, the protein of the 

 tissues is used and consequently the nitrogen of the urine 

 exceeds that in the food. 



The mini^num protein requirement therefore can be deter- 

 mined by the lowest intake which is just balanced by the 

 urinary nitrogen plus a small estimated amount for loss in 

 nitrogen excretions in faeces and in hair, hoofs, etc. 



In lactating animals account must be taken of the loss to 

 the body of the protein of the milk. In young animals, the 

 question is complicated by the growth of new tissue. 



The minimum amount of protein is not necessarily the 

 optimum amount. It is found in man, at least, that when 

 reduction of protein in the diet reaches a certain level more 

 than isodynamic quantities of fats and carbohydrates must be 

 substituted to maintain nitrogenous equilibrium, so that, on a 

 very low protein intake, the total caloric intake is increased. 

 Further, the influence on the availability of the food (p. 367), 

 and also the fact that all proteins are not of equal value 

 (p. 276), must be considered. It is usual therefore in 



